This application relies for priority upon Korean Patent Application No. 2001-09028, filed on Feb. 22, 2001, the contents of which are herein incorporated by reference in their entirety.
The present invention generally relates to a current generating circuit and, more particularly, to a current generating circuit (or constant current generating circuit) that generates a constant current using a bandgap reference circuit, regardless of resistance variation.
In a semiconductor circuit and a semiconductor memory device, a constant current generating circuit is applied to various portions. Such a constant current generating circuit is used to generate a constant current, or is employed as a current supply for a differential amplifier circuit or as a high-resistance transistor load, commonly referred to as an xe2x80x9cactive loadxe2x80x9d. For example, such a constant current generating circuit is used in a voltage-down converter in a memory device, or is used in an analog circuit such as a delay-locked loop (DLL) to shorten memory access time. Such a constant current generating circuit can be made of a bandgap reference circuit, an operational amplifier, transistors, and a resistance. Also, such a constant current generating circuit is disclosed in U.S. Pat. No. 5,519,309 entitled xe2x80x9cVOLTAGE TO CURRENT CONVERTER WITH EXTENDED DYNAMIC RANGExe2x80x9d, U.S. Pat. No. 5,629,614 entitled xe2x80x9cVOLTAGE-TO-CURRENT CONVERTERxe2x80x9d, and U.S. Pat. No. 6,087,820 entitled xe2x80x9cCURRENT SOURCExe2x80x9d.
Referring now to FIG. 1, a current generating circuit 1 acts as a voltage-to-current converter using a bandgap reference circuit. The current generating circuit 1 includes a bandgap reference circuit 10, an operational amplifier 20, two PMOS transistors M2 and M3, three NMOS transistors M1, M4, and M5, and a resistance R1. A current mirror is structured by the PMOS transistors M2 and M3, and another current mirror is structured by the NMOS transistors M4 and M5. The transistors M1-M5 operate in a saturated region. The resistance R1 is created using an N or P-type active region or polysilicon. Examples of the bandgap reference circuit shown in FIG. 1 are disclosed in U.S. Pat. No. 5,629,611 entitled xe2x80x9cCURRENT GENERATOR CIRCUIT FOR GENERATING SUBSTANTIALLY CONSTANT CURRENTxe2x80x9d and U.S. Pat. No. 6,087,820 entitled xe2x80x9cCURRENT SOURCExe2x80x9d.
Under the assumption that the operational amplifier 20 is ideal, an inversion terminal negative voltage of the operational amplifier 20 is identical to a non-inversion terminal positive voltage, i.e., a constant voltage Vbgr that is outputted from the bandgap reference circuit 10. A current I1 flowing to the resistance R1 is determined by the following equation  less than Equation 1 greater than .
I1=Vbgr/R1xe2x80x83xe2x80x83 less than Equation 1 greater than 
The current I1 also flows through the PMOS transistor M3 constituting a current mirror, and through the NMOS transistors M4 and M5 constituting another current mirror. Finally, this makes it possible to obtain a required current Iout. Since the current Iout is proportional to the current I1 flowing to the resistance R1, its variation is inversely proportional to R1, as can be seen in  less than Equation 1 greater than .
Unfortunately, the conventional current generating circuit I1 has a drawback. Since the resistance R1 is formed in a current generating circuit employing device using an active region or polysilicon, the value of R1 varies according to the fabricating process used to a degree of, for example, 10-20%. The output current Iout of the current generating circuit 1 is directly affected by such a variation. That is, if the R1 value is decreased (or increased), the current output Iout increases (or decreases), since the current Iout is inversely proportional to the decreased (or increased) resistance. As a result, the conventional current generating circuit is very sensitive to the resistance variation.
It is therefore an object of the present invention to provide a current generating circuit which is insensitive to resistance variation.
It is another object of the invention to provide a current generating circuit which generates a constant current, regardless of resistance variation.
In one aspect, the invention is directed to a current generating circuit. The current generating circuit according to the invention includes a first resistance inversely proportional current generator, a second resistance inversely proportional current generator, and a current subtractor. The first resistance inversely proportional current generator has a first resistance element and generates a first current that is inversely proportional to resistance variation of the first resistance element. The second resistance inversely proportional current generator has second and third resistance elements of the same type as the first resistance element and generates a second current that is inversely proportional to half of the resistance variation. The current subtractor subtracts the first current from the second current to generate a constant current regardless of the resistance variation of the first to third resistance elements.
In one embodiment, an intensity or density of the second current is two times larger than an intensity or density of the first current. Also, an intensity of the constant current can be identical to that of the first current.
In one embodiment, the first resistance inversely proportional current generator includes a bandgap reference circuit for generating a predetermined constant current. An operational amplifier has a non-inverse input terminal for receiving a constant voltage from the bandgap reference circuit and an inverse input terminal coupled to one end of the first resistance element and its other end grounded. Firs and second transistors are coupled to form a first current mirror. Third and fourth transistors are coupled to form a second current mirror coupled to the first current mirror. A fifth transistor has a gate for receiving an output of the operational amplifier, a source coupled to the other end of the first resistance element, and a drain coupled to the first current mirror.
In one embodiment, the second resistance inversely proportional current generator includes a first transistor having a source coupled to a power supply voltage through the second resistance element and a grounded gate. A second transistor has its source coupled to a drain of the first transistor and its gate and drain commonly grounded through the third resistance element. A third transistor has its source coupled to the drain of the first transistor, its gate coupled to the gate of the second transistor, and its drain outputs the second current. The first and third transistors constitute a current mirror circuit.
The current subtractor can include transistors constituting a current mirror circuit.
In another aspect, the invention is directed to a constant current supply including a voltage-to-current converter having a first resistance element and converting a constant voltage from a bandgap reference circuit into a first current which is inversely proportional to resistance variation of the first resistance element. The constant current supply includes a second resistance element having one end coupled to a power supply voltage, a first transistor having a source coupled to the other end of the second resistance element and a grounded gate, a second transistor having a source coupled to a drain of the first transistor and a gate and a drain which are interconnected, a third resistance element having one end coupled to the drain of the second transistor and the other end grounded, a third transistor having a source coupled to the drain of the first transistor, a gate coupled to the gate and the drain of the second transistor, and a drain for outputting a second current which is inversely proportional to half of the resistance variation, and a current subtractor subtracting the first current from the second current to output a constant current regardless of resistance variation of the first to third resistances.
In one embodiment, an intensity or density of the second current is two times larger than an intensity or density of the first current, and an intensity or density of the constant current is equal to the intensity density of the first current. The first to third resistance elements can be of the same type.